Researchers at the University of Minnesota told Iron Range workers on Friday to protect themselves from taconite dust, despite a new study that was unable to prove that exposure to the dust causes the rare cancer mesothelioma.

The study, which began in 2008, linked time spent working in the taconite industry to a higher risk of mesothelioma but stopped short of pinpointing its cause. The researchers said they have more work to do.

"No matter how you look at it, this is dusty work, and it demands that workers and employers take responsibility to safeguard themselves," said Dr. Jeff Mandel, who led the study. "Regardless of whatever else is going on with our research, you can't wait around until our results come back."

Mandel and the other researchers shared results of the Minnesota Taconite Worker Study at a meeting with Iron Range workers and their families.

"Our goal was to begin to answer questions around how mining and taconite processing have impacted the health of Minnesotans," said John Finnegan, dean of the University of Minnesota School of Public Health. "These studies have started to uncover those answers."

Mesothelioma is a form of cancer affecting the lining of the lungs. It is caused by prolonged exposure to asbestos particles in the air.

Dr. Jeff MandelDerek Montgomery for MPR

The $4.9 million study was commissioned by the Legislature in 2008 after data from the Minnesota Department of Health showed an excess of mesothelioma in men in northeastern Minnesota.

To date, University of Minnesota researchers have confirmed 80 mesothelioma deaths in Iron Range workers and also found higher rates of all types of cancer and heart disease. The study said that for every year worked in the industry, the risk for mesothelioma went up by about 3 percent.

Several current and former lawmakers representing the Iron Range were on hand for the meeting.

"What families need to know is what are the occupational hazards with the job that you have?" asked state Sen. Tom Bakk, DFL-Cook. "What is it that the person who works at the mine brings with them on their clothing at night to the family?"

Researchers said that they found no greater risk for spouses of taconite workers and said that the air in communities surrounding taconite mines has fewer particulates than air found in other parts of the state, including Minneapolis. In addition, researchers said current taconite workers are generally not exposed to unsafe levels of dust.

Joanne JohnsonDerek Montgomery for MPR

"Based on our assessment of current exposure controls, across all currently operating mines, engineering controls are appropriate," said Gurumurthy Ramachandran, a U of M environmental health sciences professor.

Ramachandran said there might be certain situations where workers are exposed to elevated dust levels. The research team looked at different job groups and compared exposure levels.

"It's probably the most comprehensive assessment of exposures done across all mines, ever," Ramachandran said. "We have made more measurements in this study than all the companies' internal databases and the MSHA [Mine Safety and Health Administration] database put together."

But the researchers did not find what caused the mesothelioma. Evaluating the dust that taconite workers were exposed to in the past proved challenging, Mandel said.

"The challenge is estimating what happened 50 to 60 years ago in the workplace," Mandel said.

He said it was also difficult to measure exposure to asbestos outside the workplace. "It is something that we want to continue to look at, if at all possible," he said.

The researchers said lifestyle is another important factor in evaluating why taconite workers have higher disease rates.

Mandel said the researchers are not finished with their investigation.

"We're going to be looking at other types of fibers, silica, respirable dust, which is the fraction that has iron particles in it, and it'll be the entirety of all those that probably will lead us to a conclusion one way or the other," he said.

About a hundred residents of the Iron Range gathered to hear the results of the study, and some in the audience questioned researchers about how it was conducted.

Bob Tammen, a retired mine worker, asked if it included enough sampling stations to measure the air quality in Iron Range communities.

"When you get a tailings pond where the beaches dry out and you get a hot dry wind, we've had communities that get tons of dust," Tammen said. "They don't have a sampling station there; they don't pick that up. So we do have situations where we don't have a complete understanding of what the mining companies are doing to our communities."

Researchers said short-term exposure was unlikely to cause health problems.

Some audience members also expressed disappointment that the study was not more conclusive. But overall the response seemed to be one of appreciation that the researchers were taking such care and were continuing to pursue the research.

"We fully understand it's about real people. This is not the last you'll year from us," said Finnegan, the university's public health dean. And he said a nurse line for concerned workers will still be available.

One of the remaining questions is whether federal air safety standards are adequate. Study authors say the air in taconite plants generally meets federal safety standards now. But those standards permit exposure to the extremely small, potentially dangerous mineral particles present in taconite facilities.

A large group of people packed the Iroquois Room at the Mountain Iron City Center during a meeting to discuss results from the Taconite Workers Health Study Friday, April 12, 2013 at the Mountain Iron Community Center in Mountain Iron, Minn.Derek Montgomery for MPRView full galleryDr. Gurumurthy Ramachandren of the University of Minnesota School of Public Health talks about results from the Taconite Workers Health Study on Friday, April 12, 2013, at the Mountain Iron Community Center in Mountain Iron, Minn.Derek Montgomery for MPR